1. Field of the Invention
This invention relates to a switch adapter in which the switch function is implemented as a single adapter. More specifically, this invention relates to a computer which can operate as a switch without damaging the function of the computer just by connecting the switch adapter to the computer.
2. Description of the Prior Art
FIG. 25 illustrates a conventional Asynchronous Transfer Mode (ATM) switch.
The conventional ATM switch is made of a single hardware frame. The inside of the ATM switch has a switch unit and a plurality of line interface units. A dedicated proprietary bus is mounted to connect the switch unit to the line interface units. A dedicated proprietary interface architecture can be utilized instead of the dedicated proprietary bus to connect the switch unit to the line interface units.
Since the ATM switch is realized as proprietary hardware, it is necessary to develop the switch according to proprietary specifications, depending on the ATM switch in a case where additional functions are implemented for the ATM switch.
FIG. 26 shows a network configuration in a case where the conventional ATM switch, as shown in FIG. 25, is utilized.
A plurality of computers and terminals are wired centrally to the ATM switch. Since a plurality of computers and terminals are wired centrally for a single ATM switch, a function for switching the ATM switch should be performed at a high speed. In addition, the number of accommodated communication lines in the line interface units of the ATM switch should be increased.
FIG. 27 shows a cell format. The cell comprises a header and a pay load. The header contains a Virtual Pass Identifier (VPI) and a Virtual Channel Identifier (VCI).
FIG. 28 shows a conventional ATM switch as shown in "B-ISDN illustration reader", page 67, Ohmu-sha, Jan. 30, 1993.
There are various controls required in the ATM switch. The basic controls are described with reference to FIG. 28 and the description of pages 66 to 71 in "B-ISDN illustration reader".
(1) routing control
The communication is started when a communication requirement called a "calling" process occurs from the terminal. In a line switching system, the "calling" process is performed according to the subscriber's line signal, such as a dial number (D channel signal in N-ISDN). In the ATM switch system, the "calling" signal is transferred to the ATM switch by the cell, found in FIG. 27. The cell that transfers the control information, such as a signal, is called as a control cell and given a predetermined VCI. This control cell can be distinguished from a cell that transmits the user information. The ATM switch receives information such as the destination to which the ATM switch is connected, types of services and a transmission speed of cells and then determines which ATM switch in the network is passed through. This procedure is called routing control.
(2) Connection Admission Control
According to the routing control, the ATM switches are connected in series. The ATM switch transfers the control cells one after another to the connected ATM switch, determined by the routing control. The ATM switch then confirms that all of the ATM switches can pass the control cells under the required service condition. For example, it judges if required traffic exceeds the transmission capacity.
When the ATM switch determines that the transmission capacity is large enough for the required traffic, it indicates that communication can be implemented for the terminal and enters a communication mode. The described communication allowing procedure is called Connection Admission Control (CAC).
(3)Usage Parameter Control
Connection Admission Control of the ATM switch differs from line switching and packet switching. The ATM switch differs because certain parameters peculiar to the ATM switch, such as transmission capacity and quality, required for transmission, are reported. Since a flow of burst traffic is allowed in the ATM switch system, transmission capacity is defined by the peak speed when the interval between cells approaches its minimum and the average speed in a long period. In the ATM switch system, management of system capacity is carried out so as to effectively handle burst traffic through use of the system equipment. When a call exceeds the quantity of contract and large quantities of cells are flown, there is a danger of lowering the quality of the whole network. Accordingly, a communication network which admits call monitors at its entrance must determine whether the transmission capacity reported by the control cell, from the transmitting terminal, matches the actual incoming quantity of cells, from the transmitting terminal.
In this method, at each contract with the subscriber, the process of discarding irregular cells, for instance, is performed when the actual traffic exceeds the predefined value. This procedure is called Usage Parameter Control (UPC).
A cell which passes through the function of UPC is transferred via the ATM switch to a route for the next node. The VCI in correspondence with the route as information required for switching is given on the header of the cell and transmitted to the next node.
(4) Shaping
Since cells are switched to each route according to the ATM switch, cells may be centralized to the specific route. As a result, there happens a case where cells should be transmitted at a fast instantaneous speed which exceeds the capacity of a Virtual Path (VP). In this case, each cell is temporarily stored in a buffer memory and read out in order that the instantaneous speed of cells might not exceed a fixed value (capacity of the VP). This procedure is called shaping.
(5) Priority control of cells
It is possible to control which cell should be transmitted with priority and then to provide a higher quality than a normal quality (the quality is shown at a cell loss rate) for the cell to be transmitted with priority. A Cell Loss Priority (CLP) bit is defined in the header of the cell, allowing user to indicate which cell should be transmitted with priority.
(6) Congestion Control
In the public network, it is important to provide the control technique such as a congestion control technique that will prevent the network from panicking when traffic is congested. There are various methods for providing a congestion control technique. For example, one method will reject the new requirement of connection according to the CAC at the time of congestion. Another method changes the parameter value of the usage parameter control, for example, restricting the peak speed at which the user can transmit.
FIGS. 29 to 31 and the following description explains the protocol based on the conventional ATM system, as shown on pages 84 to 87 of the "B-ISDN illustration reader".
A physical layer is related to the physical medium. An ATM layer deals with a transfer of the cells common to all the service. An ATM Adaptation Layer (AAL) deals with a function that depends on each service and prescribes a plurality of protocols in correspondence with each service. The addition and change of the function of the upper layer, which will depend on each service is absorbed by the ATM adaptation layer in order to prevent affecting the transferring function for cells. Protocols above the ATM adaptation layer, which will depend on the service, are all handled on the side of the terminal (This is applicable to the user information, apart from the control information to deal with the setting and release of the call).
In the ATM adaptation layer, as shown in FIG. 31, information fields (or pay loads) of several cells are connected in series. These information fields are then covered with header information on the top and trailer information on the tail. They are treated as a data unit. The ATM adaptation layer is separated into a Segmentation and Reassembly Sublayer (SARS) and a Convergence Sublayer (CS). The SARS segments the data unit into a plurality of cells and reassembles a plurality of cells into the data unit. The CS checks and testifies the correctness of data and corrects data errors depending on a requirement of each type of services. The CS then provides the result to the upper layer.
Control information deals with setting, maintenance and release of the call and connection between the network and the terminal. As in FIG. 32, the upper layer of the AAL utilizes a call control protocol based on N-ISDN.
In the figure, Q. 93B shows a B-ISDN layer 3 User Network Information (UNI) protocol, B-ISUP shows a B-ISDN User Part (B-ISDN layer 3 Network Node Interface (NNI) protocol), MTP3 is a Message Transfer Part 3 (No. 7 signal system message transfer part), S-AAL shows a Signaling-ATM adaptation Layer.
The above descriptions are quotations from the articles in "B-ISDN illustration reader", pages 66 to 71 and 84 to 87, Ohmu-sha, Jan. 30, 1993.
Since a conventional ATM switch is realized in the proprietary frame based on the specific specification, it is difficult to extend the function of the ATM switch.
For example, when the interface for a local area network (LAN) is added, it is necessary to carry out the specific development dependant on the ATM switch.
When an interface with a wide area network, like an ISDN, is adopted, it is necessary to carry out the specific development dependant on the ATM switch.
Furthermore, when a bridge function, a route function and a communication line monitoring function are added, it is necessary to carry out the specific development depending to the ATM switch.
A network configuration which utilizes a conventional ATM switch has to implement the centralized connection in which a switch is centered. As a result, problems such as the increase of the number of accommodated communication lines and the high-speeding of the processing speed of the ATM switch arise.